DR COLIN CLARK
for any other time of the year is obtained by a scaling factor based on the precipitable water depth of Central England temperature data.
The 2-year and PMP depth are then regressed against the modified Gumbel scale[40,18,22]: [1]
where T = return period (years) and y = modified Gumbel reduced variate. Note that the return period of PMP is taken as 106[35,40,18,22].
Rainfall is partitioned into slope run-off and quickflow, with the remaining rainfall becoming delayed flow.
The unit hydrograph time to peak is related to rainfall intensity and a measure of the saturated soil hydraulic conductivity as measured in the field:
where Tp = time to peak (hour) of the UH; MSL catch = mainstream channel length; MSL Brue = mainstream channel length of the upper Brue (9.0km); and c = a function of the percentage run-off at a rainfall intensity of 10mm hr-1.
Over 2500 measurements covering nearly 70 soil associations have been made. The 1:250,000 maps of the Soil Survey of England and Wales[36] are detailed enough to allow the application of data gathered elsewhere from areas with the same soil associations. To make sure that a given catchment conforms to this expectation a local survey should be carried out. In the present case the Brown Earths Rivington 2 (541g) and Neath (541h), which cover 46% of the area, were tested in the field and shown to be almost identical in hydraulic conductivity at low and moderate rainfall intensities but more responsive at higher intensities. The catchment soils and their percentage run-off are given in Table 5.
Table 5. Hydraulic conductivity of Toddbrook catchment soils 32 DAM ENGINEERING Vol XXXII Issue 1
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